Fix "absolute path" when we look to the cache directory

We should get __DIR__ instead of __DIR__ . '/.'
bug #10937 [HttpKernel] Fix "absolute path" when we look to the cache directory (BenoitLeveque)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #10937).

Discussion
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[HttpKernel] Fix "absolute path" when we look to the cache directory

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

We should get ``__DIR__`` instead of ``__DIR__ . '/.'``

I've seen this issue when trying to play with the symfony standard new directory structure

Commits
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4735e56 Fix "absolute path" when we look to the cache directory